Numerous methods and materials exist for the construction of retaining walls and landscaping walls. Such methods include the use of natural stone, poured in place concrete, masonry, and landscape timbers or railroad ties. In recent years, segmental concrete retaining wall units which are dry stacked (i.e., built without the use of mortar) have become a widely accepted product for the construction of retaining walls. Such products have gained popularity because they are mass produced, and thus relatively inexpensive. They are structurally sound, easy and relatively inexpensive to install, and couple the durability of concrete with the attractiveness of various architectural finishes.
It is desirable to build a wall from such blocks quickly and without the need for special skilled labor. The efficiency of building a wall can be measured by determining how fast the front face of a wall is constructed. Clearly, this depends on the size of the blocks used and ease of stacking the blocks.
It is standard practice in the prior art to use similarly sized mold boxes to produce various styles of block. For example, U.S. Patent Application Publication No. 2005/00161106 A1 entitled Method of Making Wall Block, the entirety of which is incorporated herein by reference, describes a standard size mold box of about 18 inches by about 24 inches (about 45.7 cm by about 61 cm), and about 8 inches (20.3 cm) deep. This standard size mold box is used to produce blocks of varying sizes. Since those blocks are typically formed in the mold with their front faces positioned along the 18 inch (45.7 cm) dimension these blocks have a front face with a dimension of 18 inches by 8 inches and a surface area of about one square foot (929 sq cm). The '106 application describes an improvement whereby two blocks are made in a standard size mold box with the front faces of the blocks formed along the 24 inch (61 cm) dimension. Those prior art blocks described in the '106 application are shown in FIG. 1. The blocks 10 are shown as they are formed within a mold box 20 and each has a front face that is about 24 inches by 8 inches (45.7 cm by 20.3 cm) and an area of about 1.33 square feet (0.124 square meter). This is larger than typical prior art blocks formed two at a time in the same size mold box which have a front face area of one square foot (0.0929 square meter). A larger front face is advantageous because more useable wall surface area is produced each mold cycle and wall construction speed and efficiency is increased because it takes fewer blocks that must be handled and aligned by laborers to build the same size wall.
During the block molding process the mold box is used to form multiple blocks at one time. The mold and a lower plate or production pallet form a cavity for the formation of blocks. Moldable material such as concrete having a composition well known to those of skill in the art is placed into the mold and allowed to set for a time sufficient to allow retention of block shape when the material is removed from the mold box. Often the blocks are formed in the mold box with their lower surface facing up and their upper surface facing down and resting against the pallet. Unless otherwise noted, that is the block orientation which is used in this application. As is well known in the art the material is removed with the aid of a compression head assembly which is lowered from above the mold box and urges the material out of the mold. Once the material is removed from the mold the material in the form and shape of a block or blocks is moved to a curing station where the blocks are allowed to cure while resting on the pallet. Another pallet is positioned under the mold to receive the moldable material which again fills the mold. In this way, many sets of multiple blocks are formed with one mold and many pallets.
During the block molding process it is important that the blocks are made of a uniform and consistent shape and size and that block dimensions, especially block height or thickness, are maintained within acceptable tolerances. This is important for all blocks but especially those made for use in dry stacked walls. There are various ways that the acceptable range of tolerance of block dimensions can be exceeded during the block molding process. Excessive wear or misalignment of the equipment and machinery used in the manufacturing process can result in the production of blocks having one or more dimensions that do not fall within acceptable tolerances. For example, irregularities in height can be the result of the blocks being formed on production pallets which have irregular surfaces. Production pallets can be made of various materials including steel, plastic and wood. Any irregularity in the surface of the production pallet will be imparted to blocks formed on that surface. Although this application focuses on problems caused by the use of fatigued and sagging production pallets it should be understood that the concepts disclosed herein are generally applicable to control tolerances and especially height/thickness tolerances of any wall block caused by any reason.
The size of a typical production pallet used in the block molding process is from 18 inches by 26 inches (46 cm by 66 cm) for the smallest pallet to 44 inches by 55 inches (112 cm by 140 cm). When the pallets are new the surface upon which the blocks are formed and cured is planar and level. The block surface resting against the pallet (typically the top surface of the block) is also planar and level since it assumes the contour of the surface of the pallet upon which it cures. However, older pallets which have been used in many production cycles can begin to sag. A block which is formed and cured on a sagging pallet or on a pallet having an irregular surface for other reasons will assume the contour of the pallet. Thus, the block will be formed with a top surface which is not planar. It is desirable that the dimensions of blocks made during this process are maintained within acceptable tolerances and that surfaces which are meant to be level are, in fact, level. This is especially true of blocks which are made with the intention that they will be dry stacked. In a wall where the blocks are connected with mortar it is possible to correct for misshapen blocks (blocks which do not fall within acceptable tolerances) by using more or less mortar. However, such correction is not possible in a dry stacked wall. If the blocks are small and the walls constructed with the blocks are not too high irregularities in block height created during the molding process may not affect use of the blocks. However, the problem is amplified in larger, wider blocks and blocks used to construct very tall walls. As discussed previously, the size and width of blocks varies depending on the size of the mold and the orientation of the blocks in the mold. For example, the width of blocks may range from less than one foot to two feet.
FIG. 2A is a front view of a prior art block 10a similar to those shown in FIG. 1. Block 10a is shown resting on a level pallet 30 while it cures. It can be seen that the top surface of block 10a which rests on the pallet is level. FIG. 2B is a front view of block 10b which is similar to the blocks shown in FIG. 1 except it is resting on a sagging pallet 40 while it cures. The drawing, which is somewhat exaggerated to make the concept clear, shows that the pallet may sag by a distance d which has been measured to be between about ⅛ inch to 3/32 inch (0.3 cm to 0.2 cm) at each end on pallets that have been in use for some time. The top surface of block 10b, which rests against the pallet, is formed with a curve or bow which results in the thickness of the block being greater at the center portion of the block than at the ends. This curve or bow in the block corresponds to the sag of the pallet causing the middle portion of the top surface to be higher than the ends by between about ⅛ inch to 3/32 inch (0.3 cm to 0.2 cm).
FIG. 3A shows a portion of a wall constructed with blocks 10a formed on a level pallet as shown in FIG. 2A. FIG. 3A shows that the thickness of the blocks is uniform and the tops and bottoms of the blocks in each course are level. The bottom surface of blocks in each course of blocks in the wall abuts against the top surface of the blocks in the next lower course without any gaps or areas of concentrated stress. This is the situation which is desired when the blocks are formed. FIG. 3B shows a portion of a wall constructed with blocks 10b formed on a sagging pallet as shown in FIG. 2B. This drawing is not to scale but is exaggerated to clearly show the increased block thickness at the middle portion of the blocks. The raised middle portion of the top surface of the blocks 10b is clearly visible. Unlike the wall of FIG. 3A the wall in FIG. 3B has areas of concentrated stress S at the top middle portion of each block in a lower course of blocks. The stress areas S are created where the raised middle portion of the top surface of the blocks contacts the blocks in the course of blocks above. FIG. 3B also shows that the portion of the block immediately below the areas of stress do not contact the blocks in the course below because that location is directly above the end portions of blocks in the lower course when the wall blocks are placed in a running bond pattern which is common when building landscape or retaining walls. The blocks are thinner at the end portions resulting in gaps between courses at those locations. Since there are gaps between the courses of blocks directly under the areas of concentrated stress there is no support provided by the underlying course of blocks at those areas. The result is that when the height of the wall is enough to create a downward force at the areas of concentrated stress S greater than the strength of the block to resist that stress without support from below a crack C can develop. The number of cracks which form in the face of the wall depends on the size of the blocks, the amount of the sag or curvature or thickness variation of the blocks, and the height of the wall. Cracks in the wall make the wall less aesthetically pleasing and, in extreme cases, if there are enough cracks can even affect the structural integrity of the wall.
Accordingly, there is a need in the art to compensate or correct for the dimensional intolerances which are created for various reasons during the block molding and curing process.